1. Field of the Invention
The present invention relates to a semiconductor storage device, and more particularly to the pattern layout of transfer transistors employed in a row decoder, which is used in a nonvolatile memory such as a NAND flash memory.
2. Description of the Related Art
A NAND flash memory is disclosed in, for example, Jin-Ki Kim et al, xe2x80x9cA 120 mm2 64 Mb NAND Flash Memory Achieving 180 ns/Byte Effective Program Speedxe2x80x9d, Symposium on VLSI Circuits, Digest of Technical Papers, pp. 168-169, 1996.
FIG. 1 illustrates a pattern layout image of a transfer transistor section provided in the row decoder of the NAND flash memory. The transfer transistor section is used to transfer, to a selected block in a memory cell array, a word-line driving signal and a selected-gate driving signal corresponding to a word line address. For facilitating the drawing and explanation, a case where eight transfer transistors are employed will be taken here as an example.
In the case of FIG. 1, the distance between broken lines Yt and Yd is determined from the size of the NAND cell, and transfer transistors Q0 to Q7 are arranged in two stages. Each transfer transistor Q0 to Q7 is an N channel MOS (NMOS) transistor formed in a p-type substrate, and its source/drain region is sufficiently resistive against a write voltage (program voltage) and an erasure voltage applied thereto.
In the arrangement of the transfer transistors Q0 to Q7 shown in FIG. 1, when executing programming, 20V+Vth (the threshold voltage of each transfer transistor), 20V, 0V and 10V are applied to the transfer transistors Q0 to Q7, a selected one of word lines WL0 to WL7, each non-selected word line adjacent to the selected one, and the other non-selected word lines, respectively. In this voltage-applied state, when writing data xe2x80x9c1xe2x80x9d (programming data xe2x80x9c1xe2x80x9d), a power supply voltage Vdd is applied to a selected bit line, while when writing data xe2x80x9c0xe2x80x9d (programming data xe2x80x9c0xe2x80x9d), a ground voltage Vss is applied to the selected bit line.
The biased state assumed when programming data is shown in FIG. 2. In the case of FIG. 2, the word line WL3 corresponding to a word-line-driving-signal CG3 is selected. The non-selected word lines adjacent to the selected word line WL3 are the word lines WL2 and WL4.
In this biased state, the distance X1 between the transfer transistors Q2 and Q3, to which word-line driving signals CG2 and CG3 are supplied, respectively, must be set at a value that enables a leak current, which occurs in a field transistor using the transistor Q3 as its drain, the transistor Q2 as its source and the gate 5 as its gate, to be kept not more than a predetermined level. Further, the distance Y1 between the transfer transistors Q3 and Q4, to which word-line driving signals CG3 and CG4 are supplied, respectively, must be set at a value that enables a leak current not more than a predetermined level to occur when 20V has been applied to an n-type diffusion region formed in the p-type substrate between element-isolating regions.
In the case of selecting another word line, the same can be said of each distance X2, X3 and Y2 to Y4.
However, in the above-described pattern layout, if the distance YB is required to be set significantly small so as to satisfy the demand for reduction of memory cell size, the transfer transistors cannot be arranged in two stages, depending upon the distance X1 or Y1 that is determined from the device design or process. In this case, a larger number of transfer transistors must be arranged in one stage, which means that the row decoder may have a significantly long length.
As described above, in the conventional semiconductor memory device, transfer transistors, employed in its row decoder for applying a write voltage or an erasure voltage to the control gate of each memory cell, must have a size sufficient to resist the write voltage and the erasure voltage. Moreover, large element-isolating regions are also needed. This being so, the pattern area of the row decoder is inevitably large.
According to an aspect of the present invention, there is provided a semiconductor device comprising: a memory cell array including a plurality of memory cells arranged in rows and columns, the memory cell array including a plurality of blocks in each one of which the memory cells are arranged; and a word-line select circuit including transfer transistors arranged in row and column directions, and configured to select at least one row of memory cells from the plurality of memory cells in a block, the word-line select circuit including: first transistors to which OV is to be applied; second transistors to which an intermediate level voltage is to be applied, the intermediate voltage being a voltage applied to a non-selected word line in a block selected in a writing operation; and third transistors to which a write voltage higher than the second voltage is to be applied, the third transistors being separated from the first transistors.